The remote accessing of computers over telephone lines is a common data transmission process. Access to the computer is normally gained by dialing a secret telephone number. As long as the secrecy of that number is maintained, unauthorized access to the computer is prevented. However, with the advent of more sophisticated computer programming and the popularity of home computers, would-be computer thiefs are finding ways to determine the secret telephone numbers.
For example, a computer can be programmed to sequentially dial every telephone number in every telephone exchange of interest. Once dialed, the computer can determine from the answer tone whether the telephone number is that of a normal user or whether a computer is being accessed thereby. Then the computer of the would-be thief can make a list of all computer-connected telephone numbers for potential unauthorized access by the thief.
The only potential solutions to this problem of which I am aware, all require the utilization of complex equipment to receive and send coded information such that in addition to accessing the computer by dialing a telephone number, certain codes must be transmitted to fully activate the communications link. As long as this code is kept secret and as long as it is complex enough that the would-be thief cannot program his computer to sequentially run through all possible codes, these systems are operable. However, the complexity of the specialized equipment involved and the need to have such equipment at the remote location and the computer location makes it cost prohibitive.